Although planetary
systems seem stable, they change over time. Wandering asteroids
occasionally hit planets and moons, and the influence of gravity,
while it may work slowly, can cause moons to collide with each
other and orbits to change. How can you set up a system that
will work long enough for your life forms to flourish?

Scientists are working with computer models to study orbits
and the relationships between planets and moons. Gravity
can affect orbits even at great distances, and with billions
of years to work, even small gravitational influences can have
power.

The force of gravity is strongest
between objects that are close to each other. Space out your
planets and moons!